Buried within the document, as The Financial Times noted on Wednesday, is a clause that essentially outlaws network level ad blocking, a relatively new technique that has seen carriers test implementing technology in their data centers to block ads served over their network as their customers use apps and the mobile web.

"ISPs [internet service providers] should not block, slow down, alter, restrict, interfere with, degrade or discriminate advertising when providing an IAS [internet access service,]" the clause, on paragraph 78 of the Body of European Regulators for Electronic Communications (BEREC) guidelines, states.

The guidelines do not restrict consumers independently choosing to download ad blocking apps from the App Store, but carriers themselves will not be able to introduce network-level ad blocking. It will be up to local telecoms regulators to implement the guidelines.

The biggest proponent of network-level ad blocking has been an Israel-based technology company called Shine. It argues that unwanted ads eat into as much as 50% of users' data plans and that the ad tech community is unfairly leaning on mobile networks' infrastructure to do the heavy lifting.

"European citizens have a right to protect themselves from being tracked, profiled and targeted by ad tech. Lobbying efforts by the advertising industry were successful in obfuscating these fundamental rights.

We are confident that Paragraph 78, that bars technology that protects from ad tech to be provided as a service, will be challenged and fail, as it infringes on the rights of European citizens to defend their private communications from unlawful tracking and profiling. Rights which are afforded to all European citizens under Ander 8 of the Charter of Fundamental Rights of the European Union and Article 5(3) of 2002/58/EC."

On O2 spokesperson said: "O2 believes in giving customers a choice. BEREC has stated that these are non-binding guidelines, with practices subject to a case-by-case assessment by National Regulatory Authorities."

For now, it looks like UK-based carriers will have to wait until after Britain's exit from the European Union to even consider a network-level ad blocking solution.

A report published earlier this year from PageFair suggested at least 419 million people worldwide are blocking ads on their smartphones, up 90% year-on-year. In March 2016 there were 14 million monthly mobile ad blocker users, according to the report.